Introduction to Classes and objects

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Introduction to Classes and objects

• Agenda
• What is an object oriented view of programming?
• What is a class?
• How to define a class?
• Problems to think about
• creating and using an object

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What is an object oriented view of programming
Suppose I want to invite my friends Tom, Liz and
Rita to meet me for coffee at Tim Hortons in the
Devonshire Mall at 8PM today. Approach 1 Give a
complete and very detailed set of instructions
on how to go from the University to Tim Hortons.
In this approach, every one exactly follows my
instructions.
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What is an object oriented view of programming
Approach 2 Simply tell my friends Pl meet me
for coffee at 8 this evening at Tim Hortons in
the Devonshire Mall Here each person follows
his/her own way of interpreting my message and
reacting to it. I dont know and I dont care the
specific mechanism used by each individual in
response to my message.
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A model for this approach

• Community of interacting agents called objects
• Each object provides a service
• All action is initiated by a message to an object

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Object oriented view

• Importance of information hiding
• Service oriented view of the world
• Why is this different from invoking a function
  in C?

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Notion of classes and objects

• Every time I use the cookie cutter, I get a
  cookie ready for baking.
• Each cookie looks exactly like the others
• View my cookie cutter as a template

My Cookie cutter
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Class - a template for similar objects

• Class definition
• What are the properties of the class?
• What are the capabilities of the class?

• Worker class
• How to characterise an object?
• name, age, place_working
• What can an object do?
• change the place of work
• describe the object as a string

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Creating an object

• First step is to define the class if needed
• Use the class as a template to create objects
• All objects of the same class has the same set of
  properties and capabilities

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Class Hierarchies and inheritance
Notion of superclass and subclasses
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Defining a class

• public class worker
• String Name // Example of a variable
• int age
• String placeWorking
• public void ChangeJob(String NewPlace) //
  Method
• placeWorking NewPlace
• // Other methods as needed
• public worker(String WorkerName) //
  Constructor
• Name WorkerName

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Static variables and methods

• Why is it useful to have class level services?
• Examples
• Number of workers
• Average salary

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How to use the services of a class

• A service gt a method
• An object of the class must exist for nonstatic
  methods
• worker w
• w new worker( Jim)//use of constructor
• Use service gt invoke right method
• w. ChangeJob(Ford, Windsor)

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Problem 1

• Define a class student.
• Think of the
• properties that characterize a student.
• capabilities of an object of class student (What
  can this student do?).

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Properties that characterize a student

• Name,
• Stud,
• DateOfBirth,
• GradeIn212,
• NumberOfCoursesTaken,
• knowsC,
• and so on

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Capabilities of objects of class student

• Change the major
• retrieve the date of birth
• compute the major grade average
• Generate a string describing the object

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Problem 2

• Define a class called course. An object of
  such a class would be the course 60-212. Remember
  that this course has been offered many times.
  Each offering has a professor, a list of students
  with their grades and so on.
• Here we may want to ask questions such as how
  many courses do we have in the University?

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Your first java application
public class First public static void
main(String args) System.out.println("60
-212 is a\npiece of cake")
Output produced 60-212 is a piece of cake
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Every application belongs to a class
This is like CLA in C . We will not use it now.
Just include it!!
public class First public static void
main(String args) System.out.println("60
-212 is a\npiece of cake")
This is how you print a message on the screen
just like printf in C
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Our second java application
import javax.swing.JOptionPane public class
Second public static void main(String
args) JOptionPane.showMessageDialog(null
, "60-212
is a\npiece of cake")
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Import in Java is just like include in C
import javax.swing.JOptionPane public class
Second public static void main(String args)
JOptionPane.showMessageDialog(null,
"60-212 is a\npiece of
cake")
From the JoptionPane class invoke the method
showMessageDialog
Display the string in the dialog window
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Points you need to remember

• General structure of a simple java program
• import classes
• public class class_name
• The name of the java program file must have
  class_name as the file name .
• The above application must be saved as
  Second.java
• JOptionPane.showMessageDialog(null, String) to
  display a string in a window

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Pseudo-program of our third problem

• main( )
• Declare variables
• Read one line of input
• Read number from the
• input line
• if (number is 0 or 1)
• factorial 1
• else while (number gt 1)
• factorial number
• decrease number by 1
• Print factorial

Read a number n Calculate n! print n!
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Our third java Application
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import javax.swing.JOptionPane public class
Third public static void main(String args)
String input_string // This string holds the
input int factorial 1, number, value
input_string JOptionPane.
showInputDialog("Enter number.

Then press OK")
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This is how you extract an integer from a string
number Integer.parseInt(input_string)
value number if ((number 0)
(number 1)) factorial 1 else
while (number gt 1) factorial
factorial number number --
JOptionPane.showMessageDialog(null,
value
"! " factorial)
In C we would have written sscanf(input_string,
d, number)
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These features are just like C
number Integer.parseInt(input_string)
value number if ((number 0)
(number 1)) factorial 1 else
while (number gt 1) factorial
factorial number number --
JOptionPane.showMessageDialog(null,
value
"! " factorial)
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Points to remember

• General structure of a simple program
• import javax.swing.JOptionPane
• public class YourClassName
• public static void main(String args)
• Body of your program

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Points to remember

• How to read an int using dialog windows?
• Read the string typed by user
• Extract integer from the string

5234
5234
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Points to remember

• How to read an int using dialog windows?
• Read the string typed by user
• Extract integer from the string
• input_string JOptionPane.showInputDialog(String)
  

Message displayed in input dialog window
number Integer.parseInt(input_string)
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Points to remember

• How to display a string using dialog windows?
• JOptionPane.showMessageDialog(null,
• String_to_be_displayed")

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Our fourth JAVA application

• Until now, we only showed one string. Now we will
  take an application where we show many lines of
  output.
• Our problem is to generate a list showing degrees
  fahrenheit and the corresponding degrees
  centigrade.
• The C program to do this is simple.
• New features shown here are
• how to specify format for floating point numbers
• how to display in a textarea

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import javax.swing. import java.text.DecimalForm
at public class Fourth public static void
main(String args) int
temp_in_fahrenheit double temp_in_celsius
DecimalFormat precisionTwo
new DecimalFormat("0.00")
JTextArea OutputArea new JTextArea(15, 30)

This area will hold lines of output
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This is how you add to the TextArea
OutputArea.append("Degrees \tDegrees\nfahrenheit\t

centigrade\n") OutputArea.append("_______
____________\n") for (temp_in_fahrenheit 0
temp_in_fahrenheit lt 100
temp_in_fahrenheit 10)
temp_in_celsius 5(temp_in_fahrenheit - 32.)/9
OutputArea.append(temp_in_fahrenheit
"\t" precisionTwo.format(temp_i
n_celsius) "\n")
JOptionPane.showMessageDialog(null, OutputArea)

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What is a textarea?
Important method append(string) adds string
to the area
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Points to remember

• To display many lines of text
• a) Define a JTextArea as follows
• JTextArea Areaname new JTextArea(NumLines,
  NumChars)
• b) Append lines of text to the text area by
• Areaname.append(A string to be shown)
• c) Instruct the text area to be displayed by
• JOptionPane.showMessageDialog(null,
• 
  Areaname)

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How to compile and execute a java application?

• For now, no command line arguments
• Step 1) type the following
• javac your_java_program
• Step 2) type the following
• java your class name
• In the case of program 3
• javac Third.java
• java Third

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Features of JAVA to remember

• Basic structure of our application is
• import javax.swing.JOptionPane
• public class Your_Class_name
• methods as needed
• public static void main(String args)
• .
• Just as you say include in C, here you import
  already existing classes.

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Problem 1
Write a Java application that will read in two
integers DepositAmount and InterestRate. The
first number represents the amount of money (in
dollars) you wish to invest for a fixed number of
years. The second number represents the interest
rate in percentage. For example, if you wish to
invest 1000 at 8 interest, the two integers
your program will read in will be 1000 and 8
respectively.
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Your program should display, in a dialog window
the money you will get back after 1, 2, ... 10
years. If P is the money you invest at interest
i for n years, the formula to calculate the money
M you get back after n years is given below M
P (1 i 0.01)n We are not interested in
displaying the fractional part of the money M.
In other words, M is an integer.
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Step 1 Read in the DepositAmount and the
InterestRate. Step 2 n 1. Step 3 While (n
lt 10), repeat steps 4 - 5 Step 4 Calculate and
display the money M you get back after n
years. Step 5 Increase n by 1.
Note Is a for statement is useful when
implementing the while loop?
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Decomposition of step 4
Calculate and display the money M you get back
after n years.
If P is the money you invest at interest i for n
years, the formula to calculate the money M you
get back after n years is given below M P
(1 i 0.01)n
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If n 1, M1 P(1 0.01 i) If n 2, M2 P(1
0.01 i)2 If n 3, M3 P(1 0.01 i)3 If n
4, M4 P(1 0.01 i)4 If n 5, M5 P(1
0.01 i)5 If n 6, M6 P(1 0.01 i)6 If n
7, M7 P(1 0.01 i)7 and so on. How to
calculate P(1 0.01 i)2 ? P(1 0.01 i)2
P(1 0.01 i) (1 0.01 i) M1 (1
0.01 i)
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If n 3, M3 P(1 0.01 i)3 How to calculate
P(1 0.01 i)3 ? M3 P(1 0.01 i)3 P(1
0.01 i)2 (1 0.01 i) M2 (1 0.01
i) Similarly, M4 M3 (1 0.01 i) and so
on Mn 1 Mn (1 0.01 i) This means we
have to go back and fix the decomposition we have
made earlier
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Our earlier decomposition
Step 1 Read in the DepositAmount and the
InterestRate. Step 2 n 1. Step 3 While (n
lt 10), repeat steps 4 - 5 Step 4 Calculate and
display the money M you get back after n
years. Step 5 Increase n by 1.
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Our new decomposition
Step 1 Read in the DepositAmount and the
InterestRate. Step 2 n 1, M
DepositAmount. Step 3 While (n lt 10), repeat
steps 4 - 6 Step 4 Calculate and display the
money Mnew you get back next year using the
formula Mnew M (1 0.01 i). Step 5
Replace M by Mnew. Step 6 Increase n by 1.
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Reminder

• General structure of a simple program
• import javax.swing.
• public class YourClassName
• public static void main(String args)
• Body of your program

This will be replaced by your decomposition
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Our Java program
import javax.swing. public class
YourClassName public static void main(String
args) Step 1 Read in
DepositAmount and InterestRate. Step 2
n 1, M DepositAmount. Step 3 While
(n lt 10), repeat steps 4 - 6 Step 4
Calculate and display the money Mnew you get back
next year using the
formula Mnew M
(1 0.01 i). Step 5 Replace M by
Mnew. Step 6 Increase n by 1.
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Reminder

• How to read an int using dialog windows?
• Read the string typed by user
• Extract integer from the string
• input_string JOptionPane.showInputDialog(String)
  

Message displayed in input dialog window
number Integer.parseInt(input_string)
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public static void main(String args)
int DepositAmount, InterestRate String
inputString inputString
JOptionPane.showInputDialog(
Type Deposit amount)
DepositAmount Integer.parseInt(input_string)
Step 1b Read in InterestRate.
Step 2 n 1, M DepositAmount.
Step 3 While (n lt 10), repeat steps 4 - 6
Step 4 Calculate and display the money
Mnew you get back
next year using the formula
Mnew M (1 0.01 i).
Step 5 Replace M by Mnew. Step 6
Increase n by 1.
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public static void main(String args)
int DepositAmount, InterestRate String
inputString inputString
JOptionPane.showInputDialog(
Type Deposit amount)
DepositAmount Integer.parseInt(input_string)
inputString JOptionPane.showInputDialog(
Type
Interest rate) InterestRate
Integer.parseInt(input_string) Step 2 n
1, M DepositAmount. Step 3 While
(n lt 10), repeat steps 4 - 6 Step 4
Calculate and display the money Mnew you get back
next year using the
formula Mnew M
(1 0.01 i). Step 5 Replace M by
Mnew. Step 6 Increase n by 1.
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public static void main(String args) int
DepositAmount, InterestRate, n, M, Mnew
String inputString inputString
JOptionPane.showInputDialog(Type Deposit
amount) DepositAmount
Integer.parseInt(input_string)
inputString JOptionPane.showInputDialog(Type
Interest rate) InterestRate
Integer.parseInt(input_string) Step 2 n
1, M DepositAmount. Step 3 While (n
lt 10), repeat steps 4 - 6 Step 4
Calculate and display the money Mnew you
get back next year using the formula
Mnew M (1 0.01 i).
Step 5 Replace M by Mnew. Step 6
Increase n by 1.
Should we use a for loop??
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public static void main(String args) int
DepositAmount, InterestRate, n, M, Mnew
String inputString inputString
JOptionPane.showInputDialog(Type Deposit
amount) DepositAmount
Integer.parseInt(input_string)
inputString JOptionPane.showInputDialog(Type
Interest rate) InterestRate
Integer.parseInt(input_string) for ( n
1, M DepositAmount n lt 10 n) Step 4
Calculate and display the money Mnew
you get back next year using the formula
Mnew M (1
0.01 i). Step 5 Replace M by Mnew.

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public static void main(String args) int
DepositAmount, InterestRate, n, M, Mnew
String inputString inputString
JOptionPane.showInputDialog(Type Deposit
amount) DepositAmount
Integer.parseInt(input_string)
inputString JOptionPane.showInputDialog(Type
Interest rate) InterestRate
Integer.parseInt(input_string) for ( n
1, M DepositAmount n lt 10 n)
Mnew M (1 0.01 i). Step 4
Display the money Mnew M Mnew.

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Reminder

• To display many lines of text
• a) Define a JTextArea as follows
• JTextArea Areaname new JTextArea(NumLines,
  NumChars)
• b) Append lines of text to the text area by
• Areaname.append(A string to be shown)
• c) Instruct the text area to be displayed by
• JOptionPane.showMessageDialog(null,
• 
  Areaname)

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public static void main(String args) int
DepositAmount, InterestRate, n, M, Mnew
String inputString JTextArea OutArea
new JTextArea(15, 20) inputString
JOptionPane.showInputDialog(Type Deposit
amount) DepositAmount
Integer.parseInt(input_string)
inputString JOptionPane.showInputDialog(Type
Interest rate) InterestRate
Integer.parseInt(input_string) for ( n
1, M DepositAmount n lt 10 n)
Mnew M (1 0.01
InterestRate) OutArea.append(n "\t"
Mnew "\n") M Mnew
JOptionPane.showMessageDialog(
null,
OutArea)
We will now compile and run the program!
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public static void main(String args) int
DepositAmount, InterestRate, n, M, Mnew
String inputString JTextArea OutArea
new JTextArea(15, 20) inputString
JOptionPane.showInputDialog(Type Deposit
amount) DepositAmount
Integer.parseInt(input_string)
inputString JOptionPane.showInputDialog(Type
Interest rate) InterestRate
Integer.parseInt(input_string) for ( n
1, M DepositAmount n lt 10 n)
Mnew M (1 0.01
InterestRate) OutArea.append(n "\t"
Mnew "\n") M Mnew
JOptionPane.showMessageDialog(
null,
OutArea)
Is there a mistake in the program?